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MG MGB Technical - Wandering valve clearances

My 74 GT (large bore) goes out of tune every thousand or so miles. Adjusting the valve clearances seems to have the greatest effect on restoring things to normal. Setting them this frequently has become a bit of a chore. The cam lift is 0.445" and double valve springs are fitted.

Am I doing something wrong? Tonight I managed to strip the thread on one of the adjuster locking nuts in my eagerness to postpone the next tuning session.

And for my further education....how do the adjusters hold together with just those itsy bitsy 1/2" nuts whilst hammering a million times a second and why don't they need washers to make them a teeny bit less likely to rattle lose? Can you tell I'm not an engineer?

Thanks,

Steve
Steve Postins

There's no need to do the locking nuts up that tight. The wandering clearances are usually caused by bad tappets that wear quickly, or valves sinking into their seats - not by the adjusters coming loose.

I'd quite like a set of those composite tappets, but I seem to remember them being a bit pricey...
--
Olly
Oliver Stephenson

Or it can be a soft cam and the lobes are shot.
Mike MaGee

Steve

Are your clearances increasing/decreasing/wandering between checks.

Increasing clearances indicate wear somewhere - cam lobes, followers, pocketing on end of rocker, rocker bush wear.

Decreasing clearances on exhaust - valve seat recession.

Wandering clearances - wear on cam followers, rocker bushes, pocketing on end of rocker.

If you have to adjust frequently, there is something wrong.


Cheers
ian f
Ian Fraser

Steve,

The itsybitsy nuts are only lock nuts - the forces of operation are taken over the 1/2 inch or so of thread in the rocker arm - the only purpose the lock nut serves is to stop the adjuster from rotating in the arm - it only needs to be nipped up, not tightened hard down. Its very action is to jam the thread of the adjuster and overtightening WILL strip threads.
Chris at Octarine Services

Thanks for the comments. I can offer a bit more info.

The clearances seem to go either way. This time about half were spot on, a couple were tight and a couple lose. I didn't take note as to which were exhaust/inlet but will do next time.

The cam and followers were new about 3000 miles ago and there was no visible evidence of exhaust valve recession at the time in the head. I had the same cycle of retuning before changing the cam and the old followers (bar one) were just about unmarked. I guess that's pointing to rocker bushes. The shaft looks nice and shiny (I'm easily impressed) but it's done around 40k.

Point well taken about the lock nuts! I always like to understand why I break things. I was lucky enough to have an old rocker assembly to hand this time but I'll be wise to this in the future.

Many thanks,

Steve
Steve Postins

What method do you use to check/adjust them - Rule of Nine? Have you tried adjusting them, spinning the engine then checking them again? For years I found mine gave inconsistent results until I discovered that some of the valves clearances are still changing at the strict 'rule of nine point' which will make repeatability very difficult. Now I turn the engine to one side or the other of the strict rule of nine point to find the point of greatest clearance and adjust to that. It has made them a bit quieter, but more importantly is consistent.
Paul Hunt

I do use the rule of nine and I do sometimes find clearances out as I go through the sequence again. I'll give it a go. Can you suggest where the best chance is to find the point of greatest gap? I realise it going to be a guess for my car but I'm keen to save any skin on the knuckles of my crank turning hand.
I remember a dwell of about 20 degrees at full lift from timing in the cam and wish I'd checked the rule of nine at the time. Next time...
Steve Postins

Are you sure you are actually getting the clearances that you think you are? The pad on the rocker arm can wear a little pocket, so that when you use a feeler gauge you are bridging the pocket, and covering the wear spot. The clearance will then be too large.
The use of a Click-adjust may be in order. Try:
http://www.mgcars.org.uk/electrical/Clikadjust_0.html
Andrew Blackley

Steve,

The top of the rocker shaft may look clean and shiny but what about the underneath ?

Slacken off the adjuster completely and slide the rocker to one side against its spring - feel underneath the shaft with your finger - ridges mean worn shaft!
Chris at Octarine Services

I'll check out the shaft when I next have the rocker cover off. I was put off the Clikadjust by the explanation at http://mgaguru.com/mgtech/tools/ca101.htm but I'll maybe reconsider.

Thanks again.
Steve Postins

To test consistancy, you could adjust the valves using the timing mark to set the piston at TDC.
Matt Kulka

Matt, that's a great idea. In fact, seeing as I took such trouble to time the cam in at 105* for full lift, I'm going to try to mark that on the crank pulley and take all the guess work out of the rule of nine method.
Steve Postins

Wear on the rocker pad will give bigger gaps than you think, but it will be consistently big making them noisy rather than finding they vary from one rotation of the crank to the next. You will have to suck it and see as to where the biggest gap is, I take the plugs out, put the car in 4th, and push or pull on the top of the road wheel moving the car to turn the engine. If your engine is like mine you will find each valve is different, meaning lots of marks on the pulley. Interesting thought, though.
Paul Hunt

Worn cam lobes should not affect valve clearances. Valve clearances are measured with the lifter on the cam base circle. FWIW, Clifton
Clifton Gordon

Paul,
Does that imply you're running an asymmetrical cam or is this just normal variation in the grind?
Steve Postins

Steve - no idea, it is only on some valves towards the back of the engine, and by different amounts. In the past I've taken everything out bar the cam to check for wear and found nothing, I just live with it.
Paul Hunt

You could use the "old school" method of checking the cleaances with the engine running. I find that the most accurate way, and there is no need to find TDC to use it. Adjusting is more difficult with the engine running, but is doable.
George B.

George,

I am interested in the engine running method. I have done it before, but only on an old Ford Bronco V-8 where I only wanted to quiet it a little and wasn't too concerned with tight tolerances. On a B do you just go to "slight noise" or something like that? Is that accurate enough? I guess you could always go back and check the gap while not running?

Dean
Dean Lake

Dean,
You still must use a feeler gauge with the engine running; you move the feeler gauge around a bit to determine when the gap is correct. If the gauge moves too freely, you're too loose and if it is difficult to move at all, you're too tight.
George B.

Hello All,

An alternative to the engine running would be to remove plugs, ensure the battery is fully charged, and have a partner crank the engine. Ensure that the spark is disconnected and remove the lead from the fuel pump. A good battery will handle this with no problem.

Cheers, Doug
Doug Keene

This thread was discussed between 16/10/2003 and 21/10/2003

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